Conversion processes employing rapid-reaction zones such as oxidation or combustion zones for the conversion of a gaseous or liquid fluid substrate in axial flow require a means for the mixing of the substrate in desired ratio with an oxidation or combustion gas for uniform contact with desired residence time within the reaction zone. Non-uniform mixing, i.e. composition, and non-uniform velocity, i.e. residence time, may lead to over or under reaction and, where this is critical, are typical of poor quality or poor yield processes. Efficient mixing may for example be achieved with use of large mixing volumes, diffuser rings placed in-stream, or of small mixing volumes operating at high superficial velocities. The mixer should moreover not introduce an unacceptable pressure differential or excessively small fluidstream dimensions which may introduce unacceptable changes in critical flow impedance by fouling or erosion.
A further hazard arises with the above conversion processes employing flammable fluids which are liable to ignition or explosion in the event of misoperation. The hazard may be considerably lessened by minimising flammable-fluid volumes in potential hazard zones and by operation at elevated superficial velocity of flammable fluids whereby residence time is less than the auto-ignition delay period. The mixing of such fluids in elevated-pressure conversion processes is possibly the single most hazardous operational stage upstream of the reaction zone, and hence the volume and superficial velocity of the mixed fluids upstream of the reaction zone is critical.
European patent application No. 656,317 contains a description of a process for the catalytic partial oxidation of a hydrocarbon feedstock in which the hydrocarbon is mixed with an oxygen-containing gas and contacted with a catalyst. The catalyst is retained in a fixed arrangement having a high tortuosity (defined as the ratio of the length of the path followed by a gas flowing through the structure to the length of the shortest possible straight line path through the structure) of at least 1.1 and having at least 750 pores per square centimetre. The catalyst preferably comprises a catalytically active metal supported on a carrier. Suitable carrier materials are described as including the refractory oxides, such as silica, alumina, titania, zirconia and mixtures thereof. A catalyst comprising a zirconia refractory foam as carrier is specifically exemplified. Comparable processes are described in European patent application Nos. 576,096 and 629,578, and in International patent application WO 96/04200.
An attractive catalytic partial oxidation process for application on a commercial scale would operate at elevated pressures, typically in excess of 10 bara, for example at around 50 bara, and at high gas hourly space velocities (normal litres of gas per kilogram of catalyst per hour), typically of the order of 20,000 to 100,000,000 Nl/kg/hr (Nl being the volume at STP (0.degree. C., 1 bara)). Due to the thermodynamic behaviour of the partial oxidation reaction, in order to obtain a high yield of carbon monoxide and hydrogen at elevated pressures, it is necessary to operate the reaction at elevated temperatures. Temperatures of the order of 800.degree. C. or higher, in some cases 1000.degree. C. or higher, are necessary for obtaining the yields demanded of a commercial process.
The operation of these processes is critical in terms of:
(1) the need to control operating conditions, uniform feedstock mixing and velocity, and catalyst and feedstock contacting, in order to substantially prevent reduced conversion to desired products due to insufficient or excess oxidation; PA1 (2) the need for low superficial velocity of feedstock at the catalyst; and PA1 (3) the need to reduce hazardous mixed feedstock volumes and residence times to minimise the chance of ignition and explosion under elevated-pressure conditions.
There is a need for a means for mixing reactive gaseous fluids in such elevated-pressure processes with uniformity of composition and velocity, to provide the desired catalyst and feedstock contacting, without increasing mixed fluid volumes or residence time in view of ignition or explosion hazard, without increasing superficial velocity to create excessive pressure differential at the catalyst, and without unacceptable changes in critical flow impedance by fouling or erosion.
Gas and fluid mixers are known, for example in partial combustion processes for the controlled combustion of hydrocarbon gases in the presence of oxygen-containing gases for acetylene production, comprising hollow conical chambers of increasing cross-sectional area in downstream direction. Diffusers are also known for the divergence of a gas or fluid stream, typically with increase of stream cross-sectional area, in manner to maintain uniformity of velocity, and to minimise recirculation (by minimising residence time) and pressure differential. For example total combustion processes employ fluid-flow diverters such as disc or tube shaped structures extending radially perpendicular to the fluid-flow path and comprising outlet apertures in the downstream disc or elongate face thereof whereby radial outward flow of fluid is encouraged by constrained outlet apertures. Also known are diffusers, comprising a hollow diffuser cone and a (manifold of) hollow rotation symmetric splitting vanes of truncated conical form thereby providing a profile to assist in maintaining and minimising the above-mentioned properties.
It will be apparent that these known devices are inadequate for the mixing and diffusion of flammable fluids, for example as required in the aforementioned conversion processes at elevated pressures which are characterised by reduced auto-ignition delay periods, and the danger of elevated-pressure explosion, bringing into conflict the constraints of the above mixers in terms of minimising ignition or explosion hazard, pressure drop, changes in critical flow impedance and optimising fluid mixing by means of uniform composition and velocity in desired ratio to obtain high quality and high yield conversion products.